Methylene chloride is a high production chemical used in a variety of applications resulting in estimated occupational and consumer exposures of at least one million people per day. Results of previously reported chronic evaluations of inhaled methylene chloride indicated that it caused mammary tumors in Fischer 344 rats and neoplasia in the lungs and liver of B6C3F1 mice. Mechanism(s) for methylene chloride-induced carcinogenesis have not been adequately elucidated. In this paper we describe the histologic evaluation of animals at a number of intermittent times for the purposes of assessing the progressive development of liver and lung neoplasia. Additionally, a series of stop-exposure treatments was conducted to evaluate the role of different methylene chloride exposure durations on the induction of hepatic and pulmonary neoplasia in female mice. Inhalation exposure to 2000 p.p.m. methylene chloride for 6 h per day, 5 days per week, for 104 weeks resulted in an 8-fold increase in the incidence of exposed animals having a lung adenoma or carcinoma (63 versus 7.5%; P < 0.01) and a 13-fold increase in the total number of pulmonary adenomas and carcinomas per animal at risk (0.97 versus 0.075; P < 0.01). This exposure also caused a 2.5-fold increase in the incidence of mice having liver tumors (69 versus 27%; P < 0.01) and a 3-fold increase in the total number of hepatic adenomas and carcinomas per animal at risk (1.34 versus 0.46; P < 0.01). Methylene chloride exposure hastened the first appearance of lung tumors (by 1 year) compared to that observed in control animals; chemical-induced and spontaneous liver tumors first occurred simultaneously. A shorter exposure duration was sufficient to attain maximal numbers of lung tumors than that needed for a maximal liver tumor burden. Lung tumor multiplicity was substantially increased by having additional time after cessation of the chemical treatment. This contrasts with the findings in liver, where additional post-exposure latency time did not effect tumor multiplicity compared to that of mice evaluated immediately after cessation of exposure. The incidence of lung alveolar hyperplasia in methylene chloride exposed animals was very low, even in tumor-bearing animals and the hyperplasias were not seen until at least 13 weeks after appearance of adenomas and carcinomas. Thus, the genesis of methylene chloride induced lung tumors in B6C3F1 mice is not preceded by overt cytotoxicity, enhanced cell proliferation nor observed hyperplasia.(ABSTRACT TRUNCATED AT 400 WORDS)